When erasing the chip, use the typical time specified in the CFI table
rather than arbitrarily selecting 5 seconds.

Since the currently unconfigurable value set in the table is 12, this
means a chip erase takes 4096 ms so this isn't a big change in behavior.

Signed-off-by: Stephen Checkoway <stephen.checkoway@oberlin.edu>
Message-Id: <20190426162624.55977-11-stephen.checkoway@oberlin.edu>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>